He wrote to Churchill in 1940 to request that Scottish Highland troops be used to install a new government in Dublin that would allow access to Irish ports for British ships.

Craigavon, a rock ribbed unionist, believed that Eamon De Valera, the Irish prime minister, had fallen under Nazi sway and a new invasion was needed.

“To meet the susceptibilities of the south the British forces might best be composed chiefly of Scottish and Welsh divisions,” he wrote in a memorandum to Churchill.

“A military governor should be then be appointed for the whole of Ireland with his HQ in Dublin.”

The British government indeed drew up such plans.

Field Marshal Montgomery noted in his memoirs: “I was told to prepare plans for the seizure of Cork and Queenstown in southern Ireland so the harbours could be used as naval bases.”

The book draws on previously classified files, obtained at the UK National Archives in London and the Public Record Office of Northern Ireland in Belfast.

Wood says a new British invasion would have backfired and resulted in a bitter war.

“British forces could have taken control with very little difficulty, but it would have an absolute gift to the IRA who would have launched waves after wave of guerrilla attacks,” he said. “Occupying Eire would have been an extremely messy and costly undertaking.”

Dr Eamon Phoenix, a political historian at Queen’s University, Belfast, also stated that it would have backfired.

“Many of the Black and Tans, the British auxiliaries sent to suppress Irish independence, were Scots and they had an appalling reputation,” he said.

Of course it has already been revealed that wily old Churchill offered De Valera a United Ireland if Ireland entered the war.